December 23, 2008

EFFECTS OF ACID RAINS, ACID FOG AND ACID MIST

Acid rains, acid fog and acid mist cause quite serious damages to natural and man-made things. These damages may be studied under following categories:

Effects on materials, buildings and man-made objects

The chemical weathering and corrosive processes of materials like coated and uncoated carbon steel, painted steel, galvanized steel, nickel-plated steel, iron, copper, nickel and other metals exposed to rains, fog and mist of acidic pH is speeded in various ways. Ferrous metals are particularly attacked by oxides of sulphur. The iron rusts, its surface becomes flaky and flakes fall off to expose more metal thus resulting in continued corrosion. As a consequence, the ion in the buildings, vehicles, railway stock and tracks, electrical and telecommunication installations etc. suffer badly. In areas having acidic rains, fog and mist, corrosion of zinc products may be ten times faster than in clean areas. Acid rain and mist also damage paint coatings and thus expose the underlying material for further damage. Acidification of surface and groundwater in the affected areas results in corrosion of submerged structures and thus submerged parts of bridges, dams, industrial equipment, water storage tanks and hydro-electric turbines are seriously damaged.

All types of buildings, especially those built of sandstone, limestone and marble are seriously damaged and their rate of decay in affected areas is often 2-3 times higher than in unaffected areas. Both stone and the mortar of buildings is affected by acid rains.

Limestone buildings are worst affected due to reaction of sulphur with calcium carbonate in presence of moisture forming calcium sulphate which is soluble and is washed away with rainwater. Soluble sulphates, nitrates, chlorides and other salts being washed over the surface of stonework crystallize within the stone when the surface water evaporates. The expansion of such salts during crystallization enlarges the cracks leading to crumbling (exfoliation) of stone surface. This, in turn, further exposes the fresh underlying stone to chemical corrosion by acid rains. Cement that has high lime content is seriously affected by acid rains while sandstone which has high silica content, is comparatively less damaged. Main visible effect of acid rains on sandstone is formation of hard, black surface coating on the exposed surface. Granite, light-coloured stones and bricks become darkened and black in acid rains. Brick-built structures are less vulnerable than stone-built structures and for this reason, historical monuments, buildings and sculptors which are mostly made up of stone and marble, are seriously damaged by acid rains. In India, acid rain damage is markedly evident on Taj Mahel of Agra, Red For and Jama Masjid of Delhi.

Continuous etching and washing away of the exterior surface of stained or unstained glass exposed to acid rains reduces the glass thickness and thus glass windows of most of most of the historical buildings in Europe are being damaged.

Effect on human health

Though wet acid deposition by acid rains has no direct effect on human health, it indirectly affects human beings. These indirect effects are not directly due to acid rain itself but are due to toxic heavy metals released by it in the environment. Acidification of soils results in release of heavy metals like Cu, Zn, Cd and Hg in the soil. These metals leach down to ground water and/or are washed down to rivers and lakes. The terrestrial plants absorb these metals from soil and aquatic plants from water. These metals thus enter the natural food chain and are passed on successively to higher trophic levels ultimately reaching human beings as plant or animal food. Human body accumulates these heavy metals over long periods of time and their concentration in human body may reach toxic levels causing various diseases. High heavy metal concentrations are known to cause osteomalacia (an uncommon bone disease)in adults and diorrhoea in babies.

Effects on freshwater aquatic flora and fauna

Dry and wet acid deposition over freshwater bodies like rivers and lakes results in serious damage to flora and fauna of these surface water bodies mainly in the following ways:

Reduction in water pH: In areas subjected to dry and wet acid deposition, unbuffered or poorly buffered surface freshwater bodies become acidified resulting in significant changes in its ecology. Critical pH level for most of the aquatic plant or animal species is 6.0. However, pH tolerance range varies amongst species as well as at different stages in the life cycle of the individuals of a species. The species that can tolerate and survive in quite wide range of pH values of water are termed tolerant species while species that can survive only within a very narrow range of water pH are termed sensitive species. Due to such differential pH sensitivity of different species and different age groups of same species, acidification of water results in marked changes in species composition, age structure and biodiversity of freshwater aquatic habitat.

Change in species composition: In freshwater bodies of normal pH, a large number of species are normally present. With increasing acidity of aquatic habitat, the species more sensitive to low pH i.e. sensitive species begin to be eliminated from the area and tolerant species are left. Gradually the acidity-sensitive species become extinct and tolerant species occupy the habitat in their place. Thus the species composition of the freshwater habitat changes due to increased acidity. For example, among animal species, many acid-sensitive species of amphibians( e.g. frogs, toads), fishes (salmon, roach and minow etc.), snails etc. become extinct and tolerant species of Hemiptera and Heteroptera (e.g. water bugs) and Corixidae (water boatman) which can survive down to a pH of 3.4, survive and expand. Among phytoplankton, most of the species of green algae, diatoms and small floating hydrophytes disappear below the pH of 5.8. Diatom species have extremely species specific narrow pH tolerance ranges and the species composition of diatoms in freshwater bodies changes very rapidly in response to pH changes of the water.

Reduction in biodiversity and food-web complexity: Most of the species of plants and animals found in freshwater ecosystem are acid-sensitive and very few of them acid-tolerant. With increasing acidification of water body, acid-sensitive species disappear and acid-tolerant species survive and spread. This results in highly reduced biodiversity in acidified freshwater bodies. Generally, the population sizes and then the number of species of angiosperm hydrophytes, algae, zooplankton, aquatic insects and fishes gradually decrease with increasing acidification of water body. The number of phytoplankton and snail species declines below pH of 5.5. Snail species completely disappear below pH of 5.2; zooplankton disappears below pH of 5.0 and fish species rapidly disappear below pH of 4.0. Though acidification affects of the animal species, impact on fish populations is quite dramatic. Many acidified lakes on Ontario in Canada have become totally fish-less while in many of the lakes, trout, wall-eye, burbot and small-mouth bass have disappeared. Many lakes above 610 meters altitude with pH below 6.0 in northeast U.S.A. have also become totally fish-less. Upto 20.000 acidified lakes in Sweden have been affected in varying degrees. About 9,000 lakes in southern Sweden and 1,400 lakes in southern Norway have few fish species left in them with roach, arctic char, trout and perch having disappeared following acidification of lakes. Generally, size and diversity of fish populations shows progressive decline below the pH of 6.0. Mass death of fish populations may also occur in lakes and rivers following acid surges induced by melting of acidified snow in upstream areas during spring season. As a result of the decrease in number of species, the complexity of food webs also decreases and the food webs gradually become simplified in acidified freshwater ecosystems.

Change in age structure and population dynamics: Breadth of pH tolerance range varies between different stages of the individuals of the same species. Young and old members of a species are often more sensitive to low pH and, therefore, they disappear more rapidly than middle-aged individuals. For example, among animals, fishes and amphibians are especially sensitive to acidity during their early embryonic stages. Thus in acidified freshwater bodies, the number of young and old individuals of the species which can tolerate acidity to some extent, gradually declines and the number of middle-aged adults gradually increases. This alters the age structure of acid-tolerant species in fresh-water ecosystems. For example, progressive decline in frog population has been reported from many acidified Swedish lakes due to inhibition of egg-hatching and death of tadpoles. Stocks of salmon have considerably declined in many acidified lakes and rivers of south and southeast Norway and western coast of Sweden. The effects of acidification on the fishes are extremely rapid. However, in low levels of acidification, older fishes survive and grow bigger in size due to reduced competition for food as a result of rapid death of younger ones, This short-term increase in fish biomass is really a signal of the imminent decline of the population due to decline in its reproductive capacity. Thus change in age structure ultimately results in changes in the population dynamics of the species ultimately leading to adverse results.

Change in rate of mineral cycling: The species of rooted hydrophytes being highly acid-sensitive, disappear while filamentous algae and moss Sphagnum being acid-tolerant, colonize the bed of acidified freshwater bodies. Fungi and bacteria that play important role in decomposition of dead organic matter are not acid-tolerant and, therefore, tend to disappear below pH of 5.5. The growth of acid-tolerant filamentous algae and moss in acidified freshwater bodies seals off the oxygen input and slows the decomposition of organic matter on the lake floors. This coupled with absence of decomposing bacteria and fungi results in very much reduced rate of decomposition of organic matter and its accumulation at the bottom of freashwater body. Thus valuable mineral nutrients become trapped in the undecomposed organic matter instead of being released again into the ecosystem by decomposition.

Increase in toxic metal ion concentration: A very damaging effect of acid rains is increase in the concentrations of heavy metals like Al, Cd, Hg, Mn, Fe and Zn in surface freshwater bodies. Acid deposition on soil and rocks in the catchement areas makes these metals in soils and rocks more soluble and mobile. Thus these released heavy metals are washed down to lakes, rivers and other surface freshwater bodies alongwith runoff water. Acidification of water bodies also mobilizes these metals from the beds into the water. These heavy metals are highly toxic to plants and animals. The metals are first taken up by aquatic plants, accumulated in their bodies and then passed on to higher trophic levels via food chains. At each level in the food chain, the concentration of toxic heavy metals increases due to their accumulation in the animal bodies over time (bio-magnification). When concentration of any metal crosses the critical threshold tolerance value in the body of an organism, it becomes toxic to that organism. Accumulation of toxic levels of metals in animal body has been show to be an important factor in reduction of population size of many aquatic species as well as predatory animals living close to water bodies.

In high concentration, Aluminium can become complexed with phosphates in the water which are often the critical limiting factors in aquatic ecosystems because they are essential nutrients for phytoplankton and hydrophytes. Reduction in phosphate leads to reduced primary production in the freshwater ecosystem. This ultimately results in progressive decrease in the food supply and, therefore, decline in population sizes of consumer animal species in higher trophic levels.

Birds like flycatchers nesting on the shores of acidic lakes eat Al-laden fish and end up with its high concentration in their bodies. Due to high Al-concentration, they produce eggs with soft or no shells and, therefore, only few eggs hatch successfully leading to decline in their population sizes. Aluminium is acutely toxic to fish at pH levels that are not normally harmful. Its concentration as low as 0.2 mg per liter kills the fishes. Though Al-poisoning interferes with normal reproduction of fishes, its more damaging effect is on the gills. Precipitation of Aluminium on the gills interferes with transport of oxygen and ions (e.g. Na+ and Cd2+) across gill membrane. Much mucus is excuded to combat the Aluminium collected on the gills which further inhibits uptake of oxygen and salts in gills. Disturbance of ionic regulation affects transport of gases between respiratory organs and the body tissues. This alongwith inhibition of oxygen-uptake, causes respiratory stress leading ultimately to death. Accumulation of Hg, Cd and Zn has also been shown to cause damage in various aquatic animal species.

Effects on terrestrial ecosystems

Acid deposition on land affects the forests and crops directly as well as indirectly through alteration of the chemistry and microbiology of soil. Though effects of acid deposition on crops have important economic consequences, the effects on forests have been very dramatic and ecologically damaging. However, the study of the effects of acid deposition on land is a very complex problem because of the following two factors:

There is a very wide range and large number of possible interactions between atmosphere, soil and plants in terrestrial ecosystems.

Effects of acid deposition on soil and vegetation take very long time (decades in case of trees) to reach detectable levels.

Despite the constraints mentioned above, studies have yielded much information about the effects of acid rains on various aspects of terrestrial ecosystems. These may be categorized as following:

Effects on soil chemistry: Following acid deposition, a series of complex chemical reactions take place in the soil. General consequences of these reactions are:

Increasing nutrient deficiency in the soil: In the acidified soil, basic cations are replaced by hydrogen and aluminium ions. These liberated cations are rapidly leached down and out of the soil solution alongwith sulphate from the acid input. Basic cations are essential plant nutrients, particularly the K+, Na+, Ca2+ and Mg2+ which are taken up by plants from the soil in quite large amounts (macronutrients). Loss of essential nutrient cations from the soil adversely affects the plant growth. Poorly buffered soils are highly susceptible to acid-induced nutrient deficiency e.g. soils of Swedish forests have shown progressively decreasing levels of K+, Na+, Ca2+ and Mg2+ over a ten-year period of acid deposition. Replacement of nutrient cations by hydrogen and aluminium ions further increases the soil acidity. Setting up a vicious cycle.

Mobilization and increase in heavy metal content of soil: Increase in soil acidity is often associated with increased soil concentration of toxic heavy metals. Most common such heavy metals are Al, Cd, Mn, Hg, Pb, Fe and Zn. In the soil of normal pH, these metals remain chemically ‘bound up’ in the soil. However, acidic pH of soil frees these metals and the mobilized metals can now rapidly spread throughout the soil alongwith natural flow of soil water.

Damage to mineral structure of soil: Soil acidification also increases the weathering of silicate minerals during liberation of metals and thus causes loss of mineral structure of the soil.

Effects on soil microbes: Acid deposition on land results in acidification of soil which causes damage to various decomposing bacterial and fungal populations in the soil. As a result, rate of decomposition of organic matter is slowed down and, therefore, the nutrient recycling in the ecosystem is blocked. Since return of essential nutrients back to the soil is blocked, the soil progressively becomes impoverished. Experimental studies have shown that soil acidity strongly reduces the decomposition of the litter of pine, spruce, birch and other cellulose-rich materials. Such reduction in decomposition of organic matter also results in reduced respiration of soil microbes including nitrogen-fixing bacteria and blue-green algae. This increases the levels of ammonia in the soil due to reduced mobilization of nutrients previously released by decomposition and the soil nitrate levels are considerably reduced due to ammonification. Such changes in the soil having pH below 3.0 bring about marked changes in the population sizes and species composition of soil microbes. For example, total abundance of acid-sensitive enchytraeids decreases and that of tolerant springtails increases. Further, soil acidification causes significant damage to other soil fauna also, particularly the earthworms. Reduced earthworm population markedly alters the soil structure and consequently the soil productivity is reduced.

Effects on terrestrial plants and ecosystem:

Effects on higher plants

All types of plants are adversely affected by acid rain and the damage is caused in two ways; firstly through shoot system, particularly the foliage which are directly exposed to acid rain, acid fog or acid mist and secondly through root system via deficiency of soil nutrients and toxicity of heavy metals in the acidified soil. Visible symptoms in plants can assume various forms depending on the character and level of acid deposition and the buffer capacity of the soil. The symptoms also vary between species and with the age of plant and tissue. Younger tissues and young plants are generally more susceptible to acid rain damage. In general, acid rain damage in plants is manifested as reduced plant growth and hence decline in yields, reduced canopy cover, reduced reproductive capacity etc.

Increased susceptibility to pathogens: Acid rains damage the surface cuticle of leaves and other plant organs and thereby make the plant more susceptible to attack by pathogenic fungi and bacteria which can now enter through the damaged surface.

Reduced growth: As discussed above, increasing soil acidity result in decreased availability of essential plant nutrients in the soil due to decreased nutrient cycling. Further, high aluminium released in soil following soil acidification has been reported to damage root hairs and thus adversely affect nutrient uptake. As a result of these, plants growing in land areas affected by acid deposition generally show poor growth. The availability of nutrients to the trees and other plants is also influenced by the exchange processes that take place on the surface of leaves. Ammonia and nitrogen landing on the leaf surface via acid deposition pass through the semi-permeable membranes of epidermal cells of leaves and are incorporated into the leaf tissue. This results in cation exchange in leaf tissues and the abundant plant nutrients present in leaf tissues such as K, Ca, Mg and S are leached and washed off the leaf surface. This foliar leaching due to acid deposition also causes depletion of essential plant nutrients and, therefore, reduced plant growth.

Foliar injury: Various visible leaf injury symptoms develop in leaves of plants growing in areas affected by acid rains. In general, visible leaf injury symptoms depend on the density of trichomes and stomata. Due to plasmolysis of palisade cells in leaves, structural damage in chloroplasts are common. In leaves of several species galls are produced in response to acid deposition.

Reduction in symbiotic balances: In the plants growing in land areas affected by acid deposition, formation of root nodules is drastically reduced and other symbiotic associations like ectotrophic and endotrophic mycorrhizae are also adversely affected.

Reduction in reproductive capacity: Decrease in flowering, reduced pollen germination, inhibition of pollen tube growth and inhibition of seed germination has been reported due to acid rains. In Norway spruce, Scots pine and Silver birth, seed germination is inhibited between pH 3.8 and 5.4. In these plants initial establishment of seedling is highly sensitive to soil pH and rapidly decreases below pH 4.2. All these effects of acid rains ultimately result in reduced reproductive success of the affected plants and, therefore, in reduced population size of the affected sensitive species. With gradual decrease in reproductive potential of affected species, the tolerant species gain upper hand and due to better reproductive success, gradually spread in the area.

Effects on lichens, algae and bryophytes

The lichens, algae and bryophytes growing on or in the soil in the areas affected by acid rains are also affected severely. Lichens drive their nutrients from the minerals falling on them with rainwater. Therefore, acid rain reduces the the availability of nutrients to lichens to a far greater extent than other plants. Rate of assimilation of nutrients in lichen thallus also varies with pH of rainwater. Further different species of lichens and bryophytic plants show different tolerance levels of rainwater acidity. Sensitive species are generally eliminated very early in the areas affected by acid rains and such areas become dominated by tolerant species of lichens and bryophytes. Thus acid rains alter the population abundance, species composition and diversity of lichen and bryophytic flora.

Effects on forests:

Damage to forests due to acid rains is a complex problem. The available evidence suggests that the damage is caused due to a combination of a variety of contributory factors in addition to acid deposition. Such factors include dry deposition of the oxides of sulphur and nitrogen, ozone, heavy metal content of soil, parasites and plant diseases, extreme climatic conditions like very high or very low rainfall and temperature extremes (particularly frost), site factors e.g. soil drainage, soil characteristics, general state of health and age of trees, surge of naturally produced acids, acid flushes (e.g. during spring snow-melt or after prolonged draught) and forest management practices. Such factors also contribute to damages to crops caused by acid rains. Ozone has been found to increase the vulnerability of trees to acid-induced damage by increasing their susceptibility to poisoning and nutrient loss. Ozone might play significant role at high altitudes where sunshine required for photo-chemical production of ozone is more intense. Vegetation above 10,000 ft. line in West Germany shows many damaged trees. Much of the damage to vegetation remains undetected until it reaches a critical, perhaps irreversible stage. Species of coniferous and deciduous trees generally exhibit much genetic variability in their populations due to which all the individuals of a species do not show equal sensitivity to acid rains. Such variability is particularly marked in Scots pine and Norway spruce. Further, different species in a forest have different dose-response relationship. All these factors make generalizations about the acid-rain induced damage to forests quite different. However, most evident effects of acid rains on forests have been observed in the form of Crown-dieback and Waldsterben.

Crown-dieback: In forest systems, damage to trees which is most extensive in West Germany, is spreading alarmingly throughout Europe and is gradually building up in U.S.A. and Canada. Visible damage tends to be concentrated in older and established trees and appears to be species-specific. Scots pine is the most sensitive species in which needles become shorter, duration of needles on the tree decreases from three to one year, top buds dry, annual growth of shoot decreases and shape of crown changes. Damaged conifers, in general, show yellowing of needles, loss of needles, distortion of branches, thinning of tree tops, injuries in bark, changes in trunks and damage to fine roots. In deciduous trees, main symptoms include discolouration of deformation of leaves, early shedding of leaves, bark injuries, death of tree tops and lack of natural regeneration. In extreme condition of damage, tree tops in all types of trees die earlier than the branches further downwards. This condition has been termed crown die-back.

Waldsterben: In 1980s, German scientists first observed the wasting disease of trees attributed to acid rains and termed it waldsterben which literally means ‘death of trees’ or ‘dying tree syndrome’ that blighted trees and forests. The extent and rate of spread of such damage is quite alarming in industrialized countries. By 1985 about 52% forest area was affected in West Germany and about 86% of woodland in East Germany showed such damage. The damage has also been found in forest of France, Switzerland, Sweden, Italy, Hungary, Poland, Czechoslowakia, Russia, U.K. Canada and U.S.A. Tree death occurs within five weeks of the appearance of first symptoms. Further, waldsterben affects young saplings as well as mature trees. In forests of areas affected by acid rains, first signs of damage were reported in Abies alba in early 1970s and in Picea abies by late 1970s. Pinus sylvestris and Fagus sylvetica were affected by early 1980s and the damage spread to other species like larch, red oak, maple, ash and rowan showing that disease affects almost all tree species. Greatest absolute damage was found in spruce and greatest relative damage occurred in silver fir in which over 87% of the trees were damaged. Three stages have been identified in this damage process:

Nitrates or nitrogen oxide in the acid rain initially provide soil nutrients and the trees grow more rapidly.

In next stage, soil progressively loses the ability to neutralize the increased acidity and the acids begin to accumulate and cause leaching of nutrient cations leading to slowing down of tree growth and yellowing or discolouration of needles or leaves. Sulphate combines with metals in soil and increases heavy metal concentration in the soil.

In the last stage, toxic aluminium is released at pH 4.2 leading to destruction of tree roots and deterioration of natural defense mechanisms of trees that prevent the entry of pathogenic bacteria, fungi and viruses. The trees thus gradually die due to nutrient deficiency, heavy metal toxicity and various pathogenic diseases.

Effects on ecosystem

Among terrestrial plants, the sequence of the sensitivity to acid rains is herbaceous dicots> woody dicots>monocots>conifers. The acid rain induced damage to trees, which are most important primary produces in the terrestrial ecosystems, reduces the food availability to animals in higher trophic levels. As a result, the population sizes of various animal species is adversely affected. In general, acid rains result in changes in relative abundance of populations in all the trophic levels and also the reduced species diversity of terrestrial ecosystems. In all the trophic levels, sensitive species are gradually eliminated and are replaced by tolerant species.

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